Slide 1

High Harmonic Fast Wave Experiments on TST-2 Y. Takase, A. Ejiri, S. Kainaga, H. Kasahara 1), R. Kumazawa 1), T. Masuda, H. Nuga, T. Oosako, M. Sasaki, Y. Shimada, F. Shimpo 1), J. Sugiyama, N. Sumitomo, H. Tojo, Y. Torii, N. Tsujii, J. Tsujimura, T. Yamada 2) 12th International Workshop on Spherical Torus 2006 Chengdu 11-13 October 2006 University of Tokyo, Kashiwa, 277-8561 Japan 1) National Institute for Fusion Science, Toki, 509-5292 Japan 2) Kyushu University, Kasuga, 816-8580 Japan Slide 2 TST-2 spherical tokamak and RF system HHFW experiment Electron heating experiment Wave diagnostics RF magnetic probes Reflectometry Wave measurements parametric decay scattering TORIC full-wave analysis EC start-up experiment Plans 200MHz experiments on TST-2 RF sustainment of high plasmas in UTST Outline Slide 3 TST-2 Spherical Tokamak ECH: 2.45GHz (< 5 kW) HHFW: 21MHz (< 200 kW x 2) ECH HHFW R / a = 0.38 / 0.25 m (A = 1.5) B t = 0.3 T / I p = 0.1 MA Slide 4 21 MHz Matching/Transmission System Slide 5 RF power 400 kW Frequency f = 13, 21, 30 MHz ( / H ~ 7 at B T = 0.2 T, f = 21 MHz) Toroidal wavenumber k = n /R 0 = 11, 16, 26 m -1 (n = 4.3, 6, 10) varied by changing the strap spacing Faraday shield angle ~ 30 current straps (0, ) Mo limiters Faraday shield Variable k Two-Strap Antenna Slide 6 Single-pass Absorption Calculation Single-pass absorption is greater for double-strap excitation Single-pass absorption increases with n e increases with T e decreases with B t (increases with e ) double-strap Slide 7 single-pass absorption = = 0.18 ELD + TTD ELD + TTD + CROSS ELD ELD + CROSS Imag ( k ) B t = 0.15 T n e = 1.010 19 m -3 T e = 100 eV n = 10 Single-Pass Absorption Improves with e Slide 8 Analysis of HHFW heating scenarios used on TST-2 is being carried out using the TORIC full-wave code. B t = 0.2 T, f = 21 MHz, n = 10, n e0 = 2 10 19 m -3, T e0 = 0.2 keV TORIC Full-Wave Calculations Electron absorption: 100% Slide 9 Soft X-ray increased, but density and radiated power did not change electron heating Strongest response near plasma center t (ms) IpIp nelnel P rad SX (> 200 eV) 360 kW RF Electron Heating by HHFW Low field side High field side ~ R 0 180kW 360kW no HHFW R=0.19m R=0.26m R=0.38m R=0.43m R=0.54m Center PS noise Slide 10 Increases in stored energy and visible-SX emission are greater for double-strap excitation Consistent with single-pass absorption calculation Single-Strap vs. Double-Strap Excitation double-strapsingle-strap no RF with RF Edge emission visible-SX emission (A.U.) P NET = 120 kW Slide 11 RF magnetic probes Sensitive to electromagnetic component Plasma edge only Reflectometry Sensitive to electrostatic component Can probe the plasma interior Both parametric decay instability (PDI) and frequency broadening due to scattering by density fluctuations were observed. These processes can alter the wavenumber spectrum, and affect both wave propagation and absorption. Wave diagnostics on TST-2 Slide 12 -60 -30 -55 -65 -115 -120 155 150 145 65 60 55 30 00 Top view S.S. enclosure Slit Core (insulator) 1-turn loop Semi-rigid cable 2cm Direction of B field to be measured RF Magnetic Probes at 14 Toroidal Locations toroidal direction BzBz BB 99 -9 -125 BB Slide 13 RF 21MHz e i t One of three sources is used Frequency sweepable VCO for profile measurements Fixed Gunn Osc. (25.85 or 27.44 GHz) for RF measurements E p x B t Ae i t Ae i t+i I cos( p + t+ RF ) sin( p + t+ RF ) VCO 6-10GHz X4 Q LO RF coaxial waveguide scalar horn Digitizer or Oscilloscope EBW Heating on TST-2@K (2003) (dW/dt) indicates P abs /P in > 50% when n e in front of antenna is steep enough Thursday: S. Shiraiwa, et al., Study of EBW Heating on TST-2 < 200 kW @ 8.2 GHz Slide 26 Typical EC Start-up Discharge (a) B t decreases gradually. (R ECH decreases gradually.) (b) I PF is kept constant. (c) P EC is kept constant. (d) I p increases with time, but disappears when the = e layer moves out of the vacuum vessel. (up to 0.5 kA produced by 4 kW) (e) n e is almost constant near the cutoff density. inboard limiter Previously achieved: 1kA/1kW (2.45GHz) 4kA/100kW (8.2GHz) Slide 27 Dependence on power and resonance position I p depends on the = e resonance position (R ECH ). I p increases with P EC, whereas n e saturates around the cutoff density 7 10 16 m -3 [NL = (5-6) 10 16 m -3 ]. time inboard limiter Slide 28 Dependence on PF Strength and Decay Index PF2+PF5 PF1 PF2PF3 1m -1m 0.1m 0.7m Mirror ratio I p maximizes at a certain field strength. Highest I p is achieved with PF2 (medium decay index). Slide 29 Effect of HHFW Injection 3 cases are compared: ECH only ECH + HHFW ECH turned off during HHFW I p responds quickly to HHFW n e and P rad increase during HHFW After ECH turn-off, I p decays and HHFW reflection increases. Slide 30 Single Particle Orbit Analysis Phase space for confined orbits is large for low A devices. Orbit-averaged toroidal precession is co-directed for all confined orbits. (c) ctr trapped(b) co trapped(a) circulating 0.10.8 m +1 Confined region in phase space (inside outermost blue boundary) (b) (a)(c) V || /V 0 V /V 0 0 1 -0.401.2 Electron orbits starting from R = 0.38m / Z = 0m Velocity is normalized by V 0 =R s p ( p is the cyclotron frequency corresponding to B p ) A. Ejiri, et al., to be published. Slide 31 Driven Current Based on Single Particle Analysis Under a low T e (or high B Z ) approximation (V Te